FCC technology, now more than 50 years old, has undergone continuous improvement and remains the predominant source of gasoline production in many refineries. This gasoline, as well as lighter products, is formed as the result of cracking heavier (i.e. higher molecular weight), less valuable hydrocarbon feed stocks such as gas oil. Although FCC is a large and complex process involving many factors, a general outline of the technology is presented here in the context of its relation to the present invention.
In its most general form, the FCC process comprises a reactor that is closely coupled with a regenerator, followed by downstream hydrocarbon product separation. Hydrocarbon feed contacts catalyst in the reactor to crack the hydrocarbons down to smaller molecular weight products. During this process, the catalyst tends to accumulate coke thereon, which is burned off in the regenerator.
The heat of combustion in the regenerator typically produces flue gas at temperatures of 718° to 760° C. (1325° to 1400° F.) and at a pressure range of 138 to 276 kPa (20 to 40 psig). Although the pressure is relatively low, the extremely high temperature, high volume of flue gas from the regenerator contains sufficient kinetic energy to warrant economic recovery. To recover energy from a flue gas stream, flue gas may be fed and directed into the blades of a power recovery expander turbine. The kinetic energy of the flue gas is transferred through the blades of the expander to a rotor coupled either to a regenerator air blower, to produce combustion air for the regenerator, and/or to a generator to produce electrical power. Because of the pressure drop of 138 to 207 kPa (20 to 30 psi) across the expander turbine, the flue gas discharges with a temperature drop of approximately 125° to 167° C. (225 to 300° F.). The flue gas may be run to a steam generator for further recovery.
The power recovery train may include an expander turbine, a generator, an air blower, a gear reducer, and a let-down steam turbine. The expander turbine may be coupled to a main air blower shaft to power the air blower of a regenerator of the FCC unit. The expander turbine is a single stage machine. The gas to the expander turbine is accelerated over a parabolic nose cone. The pressure energy is converted to kinetic energy as the flue gas passes through the blades of the turbine. The blades of the expander turbine rotate at very high velocities necessitating measures to protect the blades from physical damage.
A major distinguishing feature of an FCC process is the continuous fluidization and circulation of large amounts of catalyst having an average particle diameter of about 50 to 100 microns, equivalent in size and appearance to very fine sand. For every ton of cracked product made, approximately 5 tons of catalyst are needed, hence the considerable circulation requirements. Coupled with this need for a large inventory and recycle of catalyst with small particle diameters is the ongoing challenge to prevent this catalyst from exiting the reactor/regenerator system into effluent streams.
Catalyst particles can cause erosion of expander turbine blades resulting in loss of power recovery efficiency. Moreover, even though catalyst fines; i.e., particles less than 10 μm in dimension, do not erode expander turbine blades as significantly, they still accumulate on the blades and casing. Blade accumulation can cause blade tip erosion and casing accumulation can increase the likelihood of the tip of the blade rubbing against the casing of the expander turbine which can result in high expander shaft vibration.
Overall, the use of cyclone separators internal to both the reactor and regenerator has provided over 99% separation efficiency of solid catalyst. Typically, the regenerator includes first and second (or primary and secondary) stage separators for the purpose of preventing catalyst contamination of the regenerator flue gas, which is essentially the resulting combustion product of catalyst coke in air. While normally sized catalyst particles are effectively removed in the internal regenerator cyclones, fines material (generally catalyst fragments smaller than about 50 microns resulting from attrition and erosion in the harsh, abrasive reactor/regenerator environment) is substantially more difficult to separate. As a result, the FCC flue gas will usually contain a particulate concentration in the range of about 200 to 1000 mg/Nm3. This solids level can present difficulties related to the applicable legal emissions standards and are still high enough to risk damage to the power recovery expander turbine.
A further reduction in FCC flue gas fines loading is therefore often warranted, and may be obtained from a third stage separator (TSS). The term “third” in TSS typically presumes a first stage cyclone and a second stage cyclone are used for gas-solid separation upstream of the inlet to the TSS. These cyclones are typically located in the catalyst regeneration vessel. More or less separator devices may be used upstream of the TSS. Hence, the term TSS does not require that no more nor less than two separator devices are upstream of the TSS vessel, herein. The TSS induces centripetal acceleration to a particle-laden gas, stream to force the higher-density solids to the outer edges of a spinning vortex. To be efficient, a cyclone separator for an FCC flue gas effluent will normally contain many, perhaps 100, small individual cylindrical cyclone bodies installed within a single vessel acting as a manifold. At least one tube sheet affixing the upper and/or lower ends of the cyclones act to distribute contaminated gas to the cyclone inlets and also to divide the region within the vessel into sections for collecting the separated gas and solid phases.
Proper design of the gas delivery equipment is essential to protecting the power recovery system, particularly the blades of the expander. Cold wall piping comprises a refractory lining on the inside of a metal pipe to insulate the pipe from the hot gas carried therein to minimize thermal expansion. Cold wall piping is not typically specified between the TSS vessel and the expander turbine inlet due to concerns of spalling refractory lining entering the expander turbine and damaging the blades. Hot wall piping, which may be made of stainless steel, without refractory lining thermally expands over five times as much as cold wall piping. The large thermal expansion associated with hot wall piping systems results in significantly higher piping loads that must be accommodated in the design of the piping components and equipment. Invariably, this leads to added cost for support and installation. Additionally, the rotor of the turbo expander turbine may not be allowed to exceed a maximum velocity or the blades could detach from the rotor.
TSS vessels typically only have one main clean gas outlet in communication with the multiple main clean gas outlets of respective cyclones in the TSS vessel as shown in U.S. Pat. No. 5,690,709 and U.S. Pat. No. 5,779,746. GB 2 077 631A shows two clean gas outlets in the top hemispherical head of the TSS vessel. This reference discloses that the clean gas outlets may be connected to a power recovery turbine.